In conventional “wet” lithographic printing, ink receptive regions, known as imaged regions, are generated on a hydrophilic surface. When this surface is moistened with water and ink is also applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink can be transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket (roller) that in turn is used to transfer the ink and reproduce the image on the surface of receivers such as paper sheets, fabrics, metals, and films.
Imageable elements useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. Each imageable layer comprises the radiation-sensitive components dispersed in a polymeric binder that can respond to suitable imaging radiation. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer(s) are removed by a suitable developer or processing solution, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working, and conversely if the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer(s) that remain are ink-receptive and the regions of the hydrophilic surface revealed by development accept water and aqueous fountain solutions, and repel ink.
Such elements that are used to prepare lithographic printing plates are generally known as lithographic printing plate precursors.
In most instances, the substrate for these lithographic printing plate precursors has been an aluminum-containing support, the surface of which is generally roughened by surface graining in order to ensure good adhesion to overlying layer(s) and to improve water retention in non-imaged regions during lithographic printing. Various aluminum-containing substrates and methods for preparing them are described for example in U.S. Pat. No. 5,076,899 (Sakaki et al.) and U.S. Pat. No. 5,518,589 (Matsura et al.). To prepare aluminum-containing substrates for lithographic printing plate precursors, a continuous web of raw aluminum can be treated in a sequence of steps that are illustrated schematically in FIG. 1 of U.S. Patent Application Publication 2008/0305435 (Miyamoto) that is incorporated herein by reference. Oils are removed from the web or raw aluminum, followed by alkali etching, rinsing, graining (mechanical, electrochemical, or both), rinsing, post-graining acidic or alkali etching, rinsing, anodizing, rinsing, post-treatment, rinsing, and drying.
During anodization, the aluminum web is treated to form an aluminum oxide layer on its surface so that it will exhibit a high degree of resistance to mechanical abrasion during lithographic printing. This aluminum oxide layer is hydrophilic to some degree, but it is so reactive that it can interact with the components of the imageable layer(s) applied thereupon. The aluminum oxide layer can partially or completely cover the aluminum surface.
In the post-treatment step, the aluminum oxide layer is generally covered with a hydrophilic protective layer (also known in the art as a “seal”, “interlayer”, or “post-treatment”) to increase substrate hydrophilicity before one or more imageable layer formulations are applied. The hydrophilic post-treatment layer can be applied by immersing the web in a post-treatment coating solution or by spraying such solution onto the web using a suitable spraying and recovery system. It is desirable that the hydrophilic post-treatment layer protects the aluminum oxide layer against corrosion during development with highly alkaline developers and from dye penetration from the imageable layer(s), otherwise known as “background staining”.
U.S. Pat. No. 3,895,970 (Blum et al.) describes a phosphate/fluoride sealing process for treating metals including aluminum. In addition, EP 1,516,724 (Callant et al.) describes a process for sealing aluminum supports with phosphate/fluoride solutions to provide on-press developable lithographic printing plate precursors.
A variety of substances have been designed for use as hydrophilic post-treatment layers. Generally, such substances are polymers having carboxy, sulfonic acid, phosphonic acid, mercapto, hydroxyl, or amine functional groups. Phosphono-substituted siloxane hydrophilic protective layers are described in WO 2006/021446 (Fiebag et al.), vinyl copolymers are described for such layers in U.S. Pat. No. 7,049,048 (Hunter et al.), and copolymers having polyalkylene oxide side chains are described in WO 06/028440 (Strehmel et al.).
Other hydrophilic post-treatment layers are prepared from formulations including poly(vinyl phosphonic acid) (PVPA), vinyl phosphonic acid/acrylic acid (PVA/AA) copolymers, and poly(acrylic acid) (PAA) as described for example in U.S. Pat. No. 4,153,461 (Berghauser et al.) and U.S. Pat. No. 6,218,075 (Kimura et al.) and EP 0 537633A (Elsaesser et al). U.S. Pat. No. 4,427,765 (Mohr et al.) describes the use of a water-soluble polymer having acidic functional groups (phosphorous or sulfonic acid groups) with a salt of a divalent metal cation. U.S. Pat. No. 5,314,787 (Elsaesser et al.) describes post-treatment of aluminum substrates with a hydrophilic polymer solution followed by treatment with a solution containing divalent or polyvalent metal cations.
U.S. Patent Application Publication 2008/0305435 (noted above) describes a method for preparing aluminum substrates that includes treating the aluminum support with a post-treatment solution comprising a polymer derived from vinyl phosphonic acid and an aluminum (+3) salt and the salt and polymer concentrations are critically maintained within certain target amounts so that a certain number of phosphonic acid groups are deposited onto the anodic oxide layer of the substrate. This treatment is particularly useful when development is carried out using relatively low pH developers (for example, pH below 11.5).
EP 1,000,768A (Chau et al.) describes post-treatment of anodized aluminum supports using PVA/AA while applying a constant voltage or current to the post-treatment solution. However, such post-treatment polymers are likely to attack the aluminum oxide layer on the substrate.
U.S. Pat. No. 6,114,089 (Takita et al.) describes positive-working lithographic printing plate precursors that are considered to have improved stain resistance and press life, which improvements are believed to be provided by a polymer having both acidic and alkaline groups. The imaged precursors can be developed using silicate-free developers.
U.S. Pat. No. 5,368,974 (Walls et al.) describes post-treatment of aluminum supports using a copolymer derived from VPA and an acrylamide. However, such post-treatment polymers are likely to attack the aluminum oxide layer on the substrate.
A two-step treatment using a silicate treatment followed by a PVPA treatment is described in EP 1,974,912 (Andriessen et al.).
Post-treatment of aluminum substrates PVPA in combination with hydrogen fluoride (“HF”) to provide hydrophilic coatings is described in WO 93/15156 (Tost et al.).
The use of PVPA/HF post-treatments can cause problems during manufacturing, such as unplanned downtime to replace the post-treatment solution that is contaminated with undesired crystals. Such crystals can also clog the plumbing in the post-treatment system and require even more expensive cleaning using nitric acid. When the post-treatment solution is sprayed onto the substrate, crystal formation is avoided but several passes of the aluminum web may be needed in the spraying chamber to sufficiently cover the aluminum oxide layer and various apparatus modifications are needed to achieve uniform and complete coverage.
Despite all of these attempts to protect the aluminum oxide layer on aluminum-containing supports, there is need for further improvements in post-treatment solutions and methods so that background staining and corrosion of the aluminum oxide layer are reduced, while obtaining manufacturing efficiencies.